Environmental Chemical Data Structure and Content
This module provides an overview of the structure of the environmental chemical data and the content in the National Health and Nutrition Examination Survey (NHANES).
Task 1: Describe How the Environmental Chemical Data Fit into the NHANES Data Structure
Before you begin a search for any specific environmental chemical variables on the website, you should first understand the NHANES data structure and how environmental chemical data files fit in. This task describes how the environmental chemical data files fit into the NHANES data structure and why you need to understand NHANES data structure before you begin analyzing the data.
Task 2: Describe Environmental Chemical Data Files
The environmental chemical data are found in the Laboratory section. These laboratory data require unique collection and data processing before the data are released to the public. This task describes the steps in this process including: establishing a survey sample, defining exclusion criteria, collecting the data, processing the data, and preparing and editing the environmental chemical files.
Task 3: Describe Non-Probability Based Environmental Chemical Data Files
Some environmental analytes from NHANES were measured in a non-probability-based or "convenience” sample. For those analytes that are measured in a non-probability-based sample, care is needed when interpreting the results. Inference from these analyses is limited to those respondents in the sample and they are not representative of the U.S. population. This task explains the limitations of such samples.
Task 4: Describe the Environmental Chemical Data Subsample Files
All laboratory measurements could not be conducted for all survey respondents therefore a random subsample was selected for particular environmental components. This task describes the use of these random subsamples.
Task 5: Describe Limit of Detection Issues and Variables
For laboratory tests with a Limit of Detection (LOD), results below the LOD are replaced with a value equal to the detection limit divided by the square root of two. This value is created to help the user distinguish a nondetectable laboratory test result from a measured laboratory test result. Some environmental chemicals have values that are below the limit of detection. This task describes the value that is given for those measurements below the Limit of Detection and the variable names in the dataset.
Task 6: Describe Multi-Year Data Issues
When combining cycles of data, it is extremely important that (1) you verify that data items collected in all combined years were comparable in wording and methods and (2) you use a proper sampling weight. For the environmental chemical data, you should check the limit of detection values across cycles and check the age targets of the analytes. In order to produce estimates with greater statistical reliability, it may be necessary to combine survey cycles to increase the survey sample size. If survey cycles are combined, new weights are needed. This task describes that process.